The art relating to devices which detect an angular rate of rotation utilizing an oscillator is disclosed, for example, in Japanese Laid-Open Patent Applications No. 240,649/1993 and No. 288,555/1993 and British Patent Publication No. GB 2,266,149A.
In a device which detects an angular rate of rotation as cited above, an oscillator such as piezoelectric element is driven at its point of resonance for oscillation at a frequency which coincides with the natural frequency thereof, and a phase difference between a signal appearing at a drive terminal and a signal appearing at a detecting terminal of the oscillator is measured to detect an angular rate. However, the natural frequency of an oscillator varies under the influences of ambient temperatures or the like. Accordingly, when driving the oscillator at a given frequency in an environment which undergoes a temperature change, it is impossible to maintain the operation of the oscillator at its resonance. A deviation of the operation of the oscillator from the resonance causes a fluctuation in the amplitude of the oscillation, causing an error in the relationship between the phase difference and the angular rate.
To accommodate for this, in the cited British Patent Publication No. GB 2,266,149A, PLL (phase locked loop) circuit is employed in an attempt to maintain the operation of the oscillator at its resonance. Specifically, the frequency of the oscillation of VCO (voltage controlled oscillator) is automatically adjusted so that a signal applied to a drive terminal of the oscillator has a phase difference of 90.degree. with respect to a signal occurring at a terminal of the oscillator from which a feedback voltage is derived.
In Japanese Laid-Open Patent Application No. 240,649/1993, a pulse signal having a duty cycle which varies in accordance with a phase difference, indicative of an angular rate, is applied to an exclusive OR circuit, which passes it for smoothing and amplifying it, thus producing an analog voltage which represents the angular rate.
In Japanese Laid-Open Patent Application No. 288,555/1993, a difference in the amplitudes of signals from a pair of oscillators is amplified in a differential amplifier, the output of which is detected by a synchronous detector and smoothed, thus producing an analog voltage representing the angular rate.
In the conventional angular rate detectors, an analog circuit is adopted over the entire circuit or in major portion thereof when processing a signal or signals, and this is likely to cause an error of detection due to variations of responses from elements to elements used in the circuit or due to temperature changes.
In the device disclosed in Japanese Laid-Open Patent Application No. 240,649/1993, it is possible to employ a digital processing of a pulse signal produced by the device. By way of example, the pulse width of the signal may be counted by a counter, thus deriving information which is indicative of the angular rate. However, because the frequency of resonance of the oscillator changes with a change in the temperature, the oscillation frequency of the oscillator must be changed accordingly. A change in the oscillation frequency of the oscillator results in a change in the period of the pulse signal to be measured. This means that if the pulse signal maintains a constant duty cycle, the resulting pulse width changes. Since the angular rate corresponds to the duty cycle of the signal, it follows that a change in the oscillation frequency of the oscillator results in an error being produced in the angular rate being determined. To eliminate such an error, there must be provided a circuit which determines the period of oscillation of the oscillator, and another circuit which calculates a ratio of the period of oscillation to the pulse width, inevitably resulting in a complex arrangement. In particular, the determination of a time interval must be carried out in a rapid manner in order to increase the resolution in detecting the angular rate. However, such processing operation cannot be realized with a software in a microcomputer, presenting a need for a complex hardware arrangement.